What is the survival value of each of the following?

Researchers in the late 1950s considered three different models for the mechanism of DNA replication. In all these models, the two newly made strands are called the daughter strands, while the original strands are the parental strands. The first model is a semiconservative mechanism. Here the double-stranded DNA is half conserved following the replication process such that the new double-stranded DNA contains one parental strand and one daughter strand. This mechanism is consistent with the ideas of Watson and Crick. Even so, other models were possible and had to be ruled out. According to a second model, called conservative replication, both parental strands of DNA remain together following DNA replication. The original arrangement of parental strands is completely conserved, while the two newly made daughter strands are also together following replication. Finally, a third possibility, called a dispersive mechanism, proposed that segments of parental DNA and newly made DNA are interspersed in both strands following the replication process.



Three mechanisms for DNA replication.

The strands of the original double helix are shown in red. New strands are produced for two rounds of replication; these new strands are shown in blue.

In 1958, Matthew Meselson and Franklin Stahl devised an experimental approach to distinguish among these three possibilities. An important feature of their research was the use of isotope labelling. Nitrogen occurs in a common light (14N) form and a rare heavy (15N) form. Meselson and Stahl studied DNA replication in the bacterium E. coli. They grew E. coli cells for many generations in a medium that contained only the 15N form of nitrogen. This produced a population of bacterial cells in which all the DNA nitrogenous bases were heavy labelled. Then, they transferred the bacteria to a medium that contained only 14N as its nitrogen source. The cells were allowed to divide, and samples were collected after one round of DNA replication, two rounds, and so on. Because the bacteria were doubling in a medium that contained only 14N, all the newly made DNA strands would be labelled with light nitrogen, while the original strands would remain labelled with the heavy form.



The Meselson and Stahl experiment showing that DNA replication is semiconservative.

Meselson and Stahl used centrifugation to separate DNA molecules based on differences in density. Samples were placed on the top of a solution that contained a cesium chloride salt gradient. A double helix containing all heavy nitrogen has a higher density and will travel closer to the bottom of the gradient. By comparison, if both DNA strands contained 14N, the DNA would have a light density and remain closer to the top of the gradient. If one strand contained 14N and the other strand contained 15N, the DNA would be half-heavy and have an intermediate density, ending up near the middle of the gradient.

After one round of DNA replication, all DNA molecules exhibited a density that was half-heavy. These results are consistent with both the semiconservative and the dispersive models but not with the conservative mechanism. Because the DNA was found in a single (half-heavy) band after one doubling, the conservative model was disproved. After two rounds of replication, both light DNA and half-heavy DNA were observed. This result was also predicted by a semiconservative mechanism of DNA replication, because some DNA molecules should contain all light DNA while other molecules should be half-heavy. However, in a dispersive mechanism, all the DNA strands after two generations would have been one-fourth heavy. Taken together, the results of the Meselson and Stahl experiment are consistent only with a semiconservative mechanism for DNA replication in the prokaryote E. coli. A year later, Herbert Taylor showed that the DNA of eukaryotic chromosomal DNA also appears to be replicated se-miconservatively.

Semiconservative DNA replication relies on the complementarity of DNA strands according to the AT/GC rule. During the replication process, the two complementary strands of DNA separate and serve as template strands for the synthesis of new strands of DNA. After the double helix has separated, individual nucleotides have access to the template strands. Hydrogen bonding between individual nucleotides and the template strands must obey the AT/GC rule. A covalent bond is formed between the phosphate of one nucleotide and the sugar of the previous nucleotide. The end result is that two double helices are made that have the same base sequence as the original DNA molecule. This is a critical feature of DNA replication, because it enables the replicated DNA molecules to retain the same information (that is, the same base sequence) as the original molecule.



DNA replication according to the AT/GC rule.